Multiple wildcards on a generic methods makes Java compiler (and me!) very confused

Question

Let\'s first consider a simple scenario (see complete source on ideone.com):

import java.util.*;

public class TwoListsOfUnknowns {
    static void doNothing         


        

Answer 1

As Appendix B indicates, this has nothing to do with multiple wildcards, but rather, misunderstanding what List> really means.

Let's first remind ourselves what it means that Java generics is invariant:

1. An Integer is a Number
2. A List is NOT a List
3. A List IS a List

We now simply apply the same argument to our nested list situation _{(see appendix for more details)}:

1. A List is (captureable by) a List
2. A List> is NOT (captureable by) a List>
3. A List> IS (captureable by) a List>

With this understanding, all of the snippets in the question can be explained. The confusion arises in (falsely) believing that a type like List> can capture types like List>, List>, etc. This is NOT true.

That is, a List>:

• is NOT a list whose elements are lists of some one unknown type.
  • ... that would be a List>
• Instead, it's a list whose elements are lists of ANY type.

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Snippets

Here's a snippet to illustrate the above points:

List> lolAny = new ArrayList>();

lolAny.add(new ArrayList());
lolAny.add(new ArrayList());

// lolAny = new ArrayList>(); // DOES NOT COMPILE!!

List> lolSome;

lolSome = new ArrayList>();
lolSome = new ArrayList>();

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More snippets

Here's yet another example with bounded nested wildcard:

List> lolAnyNum = new ArrayList>();
    
lolAnyNum.add(new ArrayList());
lolAnyNum.add(new ArrayList());
// lolAnyNum.add(new ArrayList());     // DOES NOT COMPILE!!

// lolAnyNum = new ArrayList>(); // DOES NOT COMPILE!!

List> lolSomeNum;
    
lolSomeNum = new ArrayList>();
lolSomeNum = new ArrayList>();
// lolSomeNum = new ArrayList>(); // DOES NOT COMPILE!!

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Back to the question

To go back to the snippets in the question, the following behaves as expected (as seen on ideone.com):

public class LOLUnknowns1d {
    static void nowDefinitelyIllegal(List> lol, List list) {
        lol.add(list); // DOES NOT COMPILE!!!
            // The method add(capture#1-of ? extends List) in the
            // type List> is not 
            // applicable for the arguments (List)
    }
    public static void main(String[] args) {
        List